1. Primo costs twice as much and you need to use almost twice as much, so x4 the price of your injectable cycle. For me, throwing hundreds of dollars into a single cycle doesn't make sense unless I'm looking like a beast at the end! Which leads me to... 2. The gains from Primo are minimal, so if you are already a good size and just want to fill out/harden up, Mast is the best choice... 3. You can use Mast more effectively in a stack with orals or other injectables because it more effectively lowers E2... 4. I personally feel like I have more strength and aggression on Mast.
I have done two Primo cycles and one Mast cycle. To be fair, the Mast cycle was the most recent, but I don't personally see any reason to pay for Primo again. I did use DHEA and Preg on this latest cycle, which may have helped, but one of the Primo cycles was stacked with Var and I still think I like my body composition better at the end of the Mast cycle.
If money were no object, or Primo cost the same per week to run, I absolutely would choose it over Mast. However, paying almost x4 for what is almost exactly the same thing is not worth it imo.